Process for separating formaldehyde and trioxane

ABSTRACT

A process for separating a gaseous reaction mixture in the preparation of trioxane is described, in which a) the gaseous mixture leaving the reactor is scrubbed in countercurrent with an organic solvent whose boiling point is above 135° C. in which the trioxane predominantly dissolves and which leaves the formaldehyde predominantly in the gas phase which is returned to the reactor, b) the trioxane together with residual formaldehyde is stripped from the solvent by distillation via a column, the overhead product being partially condensed in the temperature range from 62° to 100° C. c) some of the resulting condensate is applied to the column as reflux and some is taken off as product and d) the non-condensed portion is returned to the scrubbing step a).

The invention relates to a process for separating mixtures offormaldehyde and trioxane, as result for example in the trimerization ofanhydrous formaldehyde in the gas phase.

Trioxane can be prepared from aqueous formaldehyde solutions using acidcatalysts. The trioxane is separated off from the reaction mixture byextraction and distillation process steps (U.S. Pat. No. 2, 304, 080,GB-B 1 012 372, DE-A 1 668 867, DE-A 1 543 815). The extraction may bereplaced by a crystallization step (DE-A 3 508 668). In these processesfor separating formaldehyde and trioxane, water is always present.

In addition, processes are known which make possible the preparation oftrioxane from formaldehyde in the gas phase (German Patent Application P42 44 582.5 of 31.12.92, German Patent Application P 43 00 138.6 of06.01.93 (each entitled "Process for the preparation of trioxane"), DE-C1 593 990, JP-A 59/25387). Trimerization in the gas phase, in additionto the higher conversion rates, has the advantage that theenergy-consuming removal of water from the reaction mixture is dispensedwith. In contrast to the separation of aqueous mixtures, a separation ofnon-aqueous mixtures has not yet been described.

The object was therefore to find a process by which the reaction mixtureof formaldehyde and trioxane can be separated in the absence of water.

The object was achieved by the invention. It describes a process forseparating a gaseous reaction mixture in the preparation of trioxane,which mixture contains as main product formaldehyde and trioxane, inwhich process a) the gaseous mixture leaving the reactor is scrubbed incountercurrent with an organic solvent whose boiling point is above 135°C. in which the trioxane predominantly dissolves and which leaves theformaldehyde predominantly in the gas phase which is returned to thereactor, b) the trioxane together with residual formaldehyde is strippedfrom the solvent by distillation via a column, the overhead productbeing partially condensed in the temperature range from 62° to 100° C.,preferably 65° to 80° C., c) some of the resulting condensate is appliedto the column as reflux and some is taken off as product and d) thenon-condensed portion is returned to the scrubbing step a).

The term "predominantly" indicates that under the conditions used inpractice complete absorption obviously does not take place or that asmall proportion of formaldehyde is always bound in the scrubbingsolvent. However, the optimal conditions can only be derived frompractice.

Solvents which are suitable are those which have a boiling point ≧135°C. and which are inert towards formaldehyde and trioxane, e.g. saturatedaliphatic and aromatic hydrocarbons and saturated aliphatic and aromaticethers and polyethers. Alkylated aromatics have proved to beadvantageous. Particularly good results have been obtained withethylbenzene, the various xylenes and diethylbenzenes.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic view of a process for separating mixtures offormaldehyde and trioxane, according to the present invention;

FIG. 2 is a schematic view illustrating the manner of determining thesolubility of formaldehyde in trioxane and the composition of theassociated gas phase; and

FIGS. 3a and b are plots illustrating that the compositions of theliquid and gas phase change only insignificantly with time.

One embodiment of the process (including the trimerization step) isoutlined in FIG. 1. The anhydrous formaldehyde (FA) is trimerized in thegas phase. The product stream (1) is scrubbed in countercurrent in thescrubber by the solvent. The trioxane substantially dissolves duringthis; the formaldehyde containing minor amounts of trioxane and solventis returned (2) to the trimerization reactor. After leaving the scrubberthe solvent containing the dissolved trioxane (3) is separated in adistillation column. The bottom stream (7) is freed of trioxane and isreused for the scrubber. The overhead product is partially condensed(temperature range 62° to 100° C., preferably 65° to 80° C.). Thenon-condensed portions are returned (6) to the scrubber; of thecondensed portions, a part-stream is applied (4) as reflux to thedistillation column and the other part-stream is taken off as productstream (5). If necessary, this stream (5) can be finely purified byfurther distillation steps, for example to separate off the residualdissolved formaldehyde.

Experimental description

Two series of experiments were carried out.

EXAMPLE 1

The solubility of formaldehyde in trioxane and the composition of theassociated gas phase were determined in equipment as in FIG. 2. Trioxanewas introduced into a thermostated double-walled vessel having a bottomdischarge. Formaldehyde, prepared by decomposition ofcyclohexylhemiformal, was passed through molten trioxane at 70° C.During the experimental period of 140 minutes, samples of trioxane weretaken via the bottom discharge 4 and collected in ten times the volumeof water. The samples were analyzed by gas chromatography; 1.3 to 1.7per cent by mass formaldehyde were found in the trioxane. The gas streamwas collected in water. In the water at the end of the experimentformaldehyde and trioxane were found in a ratio of 1:1. Thisexperimental arrangement can serve as a model for the partialcondensation of the overhead stream of the distillation column(compositions of the streams (4) and (6)).

EXAMPLES 2 to 12

In these experiments the equilibrium compositions which are establishedduring scrubbing with the solvent were determined. The arrangementcorresponds to that of Example 1 (FIG. 2). In these experiments, thesolvent was introduced in a mixture with trioxane into the thermostatedvessel. Formaldehyde was introduced over an experimental period of 5hours. Samples of the liquid were taken and the water which serves asabsorption medium was changed in an hourly cycle. Both the water and thesamples of the organic solvent were analyzed by gas chromatography. Asis shown in FIGS. 3a and b, the compositions of the gas phase and of theliquid change only insignificantly with time. The compositions averagedover time which were established in Examples 2 to 12 are summarized inTable 1. The solvent, the solvent temperature and the solvent loadingwith trioxane were changed. This experimental arrangement can serve as amodel for scrubbing with the solvent (compositions of the streams (2)and (3)).

                                      TABLE 1    __________________________________________________________________________    Example     Temperature                      Liquid  percent by mass!                                      Gas phase  percent by                                                      Separation    number         Solvent                 °C.!                      Formaldehyde                             Trioxane                                  Solvent                                      Formaldehyde                                             Trioxane                                                  Solvent                                                      factor.sup.1)    __________________________________________________________________________    2    Ethylbenzene                32    1.8    13.9 84.3                                      90.9   3.0  6.1 234    3    Ethylbenzene                40    1.5    14.0 84.5                                      84.4   5.3  10.3                                                      148    4    Ethylbenzene                50    1.4    12.5 86.1                                      80.9   7.0  12.1                                                      103    5    Xylene.sup.2)                25    1.9    13.5 84.6                                      91.6   3.0  5.4 217    6    Xylene 32    1.3    15.0 83.7                                      80.3   3.9  15.8                                                      237    7    Xylene 40    1.5    12.3 86.2                                      85.3   6.5  8.2 108    8    Xylene 50    0.9    12.0 87.1                                      77.6   8.9  8.9 117    9    Xylene 50    1.1    22.2 76.3                                      71.8   14.7 12.5                                                       99    10   Xylene 50    1.7    28.7 69.6                                      75.8   15.2 9.0  84    11   Diethyl-                32    1.5    14.5 84.0                                      92.6   3.2  4.2 280         benzene.sup.3)    12   Diethyl-                50    1.2    23.4 75.4                                      77.3   12.9 9.8 116         benzene    __________________________________________________________________________     .sup.1) Separation factor = (TOX.sub.1 *FA.sub.g)/(TOX.sub.g *FA.sub.1)     .sup.2) Mixture of isomers of o,pxylene     .sup.3) Mixture of isomers of o,pdiethylbenzene

We claim:
 1. A process for separating a gaseous reaction mixture in thepreparation of trioxane, which mixture contains as main productformaldehyde and trioxane, In which process a) the gaseous mixtureleaving the reactor is scrubbed in countercurrent with an organicsolvent whose boiling point is above 135° C. in which the trioxanepredominantly dissolves and which leaves the formaldehyde predominantlyin the gas phase which is returned to the reactor, b) the trioxanetogether with residual formaldehyde is stripped from the solvent bydistillation via a column, the overhead product being partiallycondensed in the temperature range from 62° to 100° C. c) some of theresulting condensate is applied to the column as reflux and some istaken off as product and d) the non-condensed portion is returned to thescrubbing step a).
 2. The process as claimed in claim 1, wherein thetemperature range is 65° to 80° C.
 3. The process as claimed in claim 1,wherein the solvent used is saturated aliphatic and aromatichydrocarbons or aliphatic and aromatic ethers and polyethers.
 4. Theprocess as claimed in claim 1, wherein the solvent used is ethylbenzene,the various xylenes or diethylbenzenes are used.